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<h1 class="lined-header"><span>cowboy_stream(3)</span></h1>
<h2 id="_name">Name</h2>
<p>cowboy_handler - Stream handlers</p>
<h2 id="_description">Description</h2>
<p>The module <code>cowboy_stream</code> defines a callback interface and a protocol for handling HTTP streams.</p>
<p>An HTTP request and its associated response is called a stream. A connection may have many streams. In HTTP/1.1 they are executed sequentially, while in HTTP/2 they are executed concurrently.</p>
<p>Cowboy calls the stream handler for nearly all events related to a stream. Exceptions vary depending on the protocol.</p>
<p>Extra care must be taken when implementing stream handlers to ensure compatibility. While some modification of the events and commands is allowed, it is generally not a good idea to completely omit them.</p>
<h2 id="_callbacks">Callbacks</h2>
<p>Stream handlers must implement the following interface:</p>
<div class="listingblock"><div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><b><font color="#000000">init</font></b>(<font color="#009900">StreamID</font>, <font color="#009900">Req</font>, <font color="#009900">Opts</font>) <font color="#990000">-></font> {<font color="#009900">Commands</font>, <font color="#009900">State</font>}
<b><font color="#000000">data</font></b>(<font color="#009900">StreamID</font>, <font color="#009900">IsFin</font>, <font color="#009900">Data</font>, <font color="#009900">State</font>) <font color="#990000">-></font> {<font color="#009900">Commands</font>, <font color="#009900">State</font>}
<b><font color="#000000">info</font></b>(<font color="#009900">StreamID</font>, <font color="#009900">Info</font>, <font color="#009900">State</font>) <font color="#990000">-></font> {<font color="#009900">Commands</font>, <font color="#009900">State</font>}
<b><font color="#000000">terminate</font></b>(<font color="#009900">StreamID</font>, <font color="#009900">Reason</font>, <font color="#009900">State</font>) <font color="#990000">-></font> <b><font color="#000000">any</font></b>()
<b><font color="#000000">early_error</font></b>(<font color="#009900">StreamID</font>, <font color="#009900">Reason</font>, <font color="#009900">PartialReq</font>, <font color="#009900">Resp</font>, <font color="#009900">Opts</font>) <font color="#990000">-></font> <font color="#009900">Resp</font>
<font color="#009900">StreamID</font> <font color="#990000">::</font> <b><font color="#000000">cowboy_stream:streamid</font></b>()
<font color="#009900">Req</font> <font color="#990000">::</font> <b><font color="#000000">cowboy_req:req</font></b>()
<font color="#009900">Opts</font> <font color="#990000">::</font> <b><font color="#000000">cowboy:opts</font></b>()
<font color="#009900">Commands</font> <font color="#990000">::</font> <b><font color="#000000">cowboy_stream:commands</font></b>()
<font color="#009900">State</font> <font color="#990000">::</font> <b><font color="#000000">any</font></b>()
<font color="#009900">IsFin</font> <font color="#990000">::</font> <b><font color="#000000">cowboy_stream:fin</font></b>()
<font color="#009900">Data</font> <font color="#990000">::</font> <b><font color="#000080">binary</font></b>()
<font color="#009900">Info</font> <font color="#990000">::</font> <b><font color="#000000">any</font></b>()
<font color="#009900">Reason</font> <font color="#990000">::</font> <b><font color="#000000">cowboy_stream:reason</font></b>()
<font color="#009900">PartialReq</font> <font color="#990000">-</font> <b><font color="#000000">cowboy_req:req</font></b>(), <font color="#FF6600">except</font> <font color="#FF6600">all</font> <font color="#FF6600">fields</font> <font color="#FF6600">are</font> <font color="#FF6600">optional</font>
<font color="#009900">Resp</font> <font color="#990000">::</font> <b><font color="#000000">cowboy_stream:resp_command</font></b>()</tt></pre>
</div></div>
<p>HTTP/1.1 will initialize a stream only when the request-line and all headers have been received. When errors occur before that point Cowboy will call the callback <code>early_error/5</code> with a partial request, the error reason and the response Cowboy intends to send. All other events go throuh the stream handler using the normal callbacks.</p>
<p>HTTP/2 will initialize the stream when the <code>HEADERS</code> block has been fully received and decoded. Any protocol error occuring before that will not result in a response being sent and will therefore not go through the stream handler. In addition Cowboy may terminate streams without sending an HTTP response back.</p>
<p>The stream is initialized by calling <code>init/3</code>. All streams that are initialized will eventually be terminated by calling <code>terminate/3</code>.</p>
<p>When Cowboy receives data for the stream it will call <code>data/4</code>. The data given is the request body after any transfer decoding has been applied.</p>
<p>When Cowboy receives a message addressed to a stream, or when Cowboy needs to inform the stream handler that an internal event has occurred, it will call <code>info/3</code>.</p>
<h2 id="commands">Commands</h2>
<p>Stream handlers can return a list of commands to be executed from the <code>init/3</code>, <code>data/4</code> and <code>info/3</code> callbacks. In addition, the <code>early_error/5</code> callback must return a response command.</p>
<!-- @todo We need a 'log' command that would call error_logger.-->
<!-- It's better than doing in the handlers directly because-->
<!-- then we can have other stream handlers manipulate those logs.-->
<!-- @todo We need a command to send a message so that other-->
<!-- stream handlers can manipulate these messages if necessary.-->
<p>The following commands are defined:</p>
<h3 id="inform_command">inform</h3>
<p>Send an informational response to the client.</p>
<div class="listingblock"><div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt>{<font color="#FF6600">inform</font>, <b><font color="#000000">cowboy:http_status</font></b>(), <b><font color="#000000">cowboy:http_headers</font></b>()}</tt></pre>
</div></div>
<p>Any number of informational responses may be sent, but only until the final response is sent.</p>
<h3 id="response_command">response</h3>
<p>Send a response to the client.</p>
<div class="listingblock"><div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt>{<font color="#FF6600">response</font>, <b><font color="#000000">cowboy:http_status</font></b>(), <b><font color="#000000">cowboy:http_headers</font></b>(),
<b><font color="#000000">cowboy_req:resp_body</font></b>()}</tt></pre>
</div></div>
<p>No more data can be sent after this command.</p>
<h3 id="headers_command">headers</h3>
<p>Initiate a response to the client.</p>
<div class="listingblock"><div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt>{<font color="#FF6600">headers</font>, <b><font color="#000000">cowboy:http_status</font></b>(), <b><font color="#000000">cowboy:http_headers</font></b>()}</tt></pre>
</div></div>
<p>This initiates a response to the client. The stream will end when a data command with the <code>fin</code> flag or a trailer command is returned.</p>
<h3 id="data_command">data</h3>
<p>Send data to the client.</p>
<div class="listingblock"><div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt>{<font color="#FF6600">data</font>, <b><font color="#000000">fin</font></b>(), <b><font color="#000000">iodata</font></b>()}</tt></pre>
</div></div>
<h3 id="trailers_command">trailers</h3>
<p>Send response trailers to the client.</p>
<div class="listingblock"><div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt>{<font color="#FF6600">trailers</font>, <b><font color="#000000">cowboy:http_headers</font></b>()}</tt></pre>
</div></div>
<h3 id="push_command">push</h3>
<p>Push a resource to the client.</p>
<div class="listingblock"><div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt>{<font color="#FF6600">push</font>, <font color="#009900">Method</font>, <font color="#009900">Scheme</font>, <font color="#009900">Host</font>, <b><font color="#000000">inet:port_number</font></b>(),
<font color="#009900">Path</font>, <font color="#009900">Qs</font>, <b><font color="#000000">cowboy:http_headers</font></b>()}
<font color="#009900">Method</font> <font color="#990000">=</font> <font color="#009900">Scheme</font> <font color="#990000">=</font> <font color="#009900">Host</font> <font color="#990000">=</font> <font color="#009900">Path</font> <font color="#990000">=</font> <font color="#009900">Qs</font> <font color="#990000">=</font> <b><font color="#000080">binary</font></b>()</tt></pre>
</div></div>
<p>The command will be ignored if the protocol does not provide any server push mechanism.</p>
<h3 id="_flow">flow</h3>
<div class="listingblock"><div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt>{<font color="#FF6600">flow</font>, <b><font color="#000000">pos_integer</font></b>()}</tt></pre>
</div></div>
<p>Request more data to be read from the request body. The exact behavior depends on the protocol.</p>
<h3 id="_spawn">spawn</h3>
<p>Inform Cowboy that a process was spawned and should be supervised.</p>
<div class="listingblock"><div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt>{<b><font color="#000080">spawn</font></b>, <b><font color="#000080">pid</font></b>(), <b><font color="#000000">timeout</font></b>()}</tt></pre>
</div></div>
<h3 id="_error_response">error_response</h3>
<p>Send an error response if no response was sent previously.</p>
<div class="listingblock"><div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt>{<font color="#FF6600">error_response</font>, <b><font color="#000000">cowboy:http_status</font></b>(), <b><font color="#000000">cowboy:http_headers</font></b>(), <b><font color="#000000">iodata</font></b>()}</tt></pre>
</div></div>
<h3 id="switch_protocol_command">switch_protocol</h3>
<p>Switch to a different protocol.</p>
<div class="listingblock"><div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt>{<font color="#FF6600">switch_protocol</font>, <b><font color="#000000">cowboy:http_headers</font></b>(), <b><font color="#000000">module</font></b>(), <b><font color="#000000">state</font></b>()}</tt></pre>
</div></div>
<p>Contains the headers that will be sent in the 101 response, along with the module implementing the protocol we are switching to and its initial state.</p>
<h3 id="_stop">stop</h3>
<p>Stop the stream.</p>
<div class="listingblock"><div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><font color="#FF6600">stop</font></tt></pre>
</div></div>
<p>While no more data can be sent after the <code>fin</code> flag was set, the stream is still tracked by Cowboy until it is stopped by the handler.</p>
<p>The behavior when stopping a stream for which no response has been sent will vary depending on the protocol. The stream will end successfully as far as the client is concerned.</p>
<p>To indicate that an error occurred, either use <code>error_response</code> before stopping, or use <code>internal_error</code>.</p>
<h3 id="_internal_error">internal_error</h3>
<p>Stop the stream with an error.</p>
<div class="listingblock"><div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt>{<font color="#FF6600">internal_error</font>, <font color="#009900">Reason</font>, <font color="#009900">HumanReadable</font>}
<font color="#009900">Reason</font> <font color="#990000">=</font> <b><font color="#000000">any</font></b>()
<font color="#009900">HumanReadable</font> <font color="#990000">=</font> <b><font color="#000080">atom</font></b>()</tt></pre>
</div></div>
<p>This command should be used when the stream cannot continue because of an internal error. An <code>error_response</code> command may be sent before that to advertise to the client why the stream is dropped.</p>
<h2 id="_predefined_events">Predefined events</h2>
<p>Cowboy will forward all messages sent to the stream to the <code>info/3</code> callback. To send a message to a stream, send a message to the connection process with the form <code>{{Pid, StreamID}, Msg}</code>. The connection process will then forward <code>Msg</code> to the stream handlers.</p>
<p>Cowboy will also forward the exit signals for the processes that the stream spawned.</p>
<h3 id="_exit">EXIT</h3>
<!-- info(_StreamID, {'EXIT', Pid, normal}, State=#state{pid=Pid}) ->-->
<!-- info(_StreamID, {'EXIT', Pid, {_Reason, [_, {cow_http_hd, _, _, _}|_]}}, State=#state{pid=Pid}) ->-->
<!-- info(StreamID, Exit = {'EXIT', Pid, {Reason, Stacktrace}}, State=#state{ref=Ref, pid=Pid}) ->-->
<p>A process spawned by this stream has exited.</p>
<div class="listingblock"><div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt>{<font color="#000080">'EXIT'</font>, <b><font color="#000080">pid</font></b>(), <b><font color="#000000">any</font></b>()}</tt></pre>
</div></div>
<p>This is the raw exit message without any modification.</p>
<!-- === read_body-->
<!-- -->
<!-- //info(_StreamID, {read_body, Ref, Length, _},-->
<!-- //info(StreamID, {read_body, Ref, Length, Period}, State) ->-->
<!-- -->
<!-- TODO yeah I am not actually sure this one should be public just yet-->
<!-- TODO if it is, then we probably shouldn't send a message directly,-->
<!-- TODO but rather return a command that will end up sending the message-->
<!-- -->
<!-- TODO The problem being that no stream handler has access to that-->
<!-- TODO message if we send it directly. So we should have a command-->
<!-- TODO send_message or something that can be seen from all handlers.-->
<!-- -->
<!-- TODO The thing is that stream handlers can have 0 to N processes-->
<!-- TODO so we have to make it easy to say which process should-->
<!-- TODO receive the message, and perhaps *identify* which process-->
<!-- TODO gets it?-->
<h3 id="_inform">inform</h3>
<p>Same as the <a href="../inform_command">inform command</a>.</p>
<p>Sent when the request process reads the body and an expect: 100-continue header was present in the request, or when the request process sends an informational response on its own.</p>
<h3 id="_response">response</h3>
<p>Same as the <a href="../response_command">response command</a>.</p>
<p>Usually sent when the request process replies to the client. May also be sent by Cowboy internally.</p>
<h3 id="_headers">headers</h3>
<p>Same as the <a href="../headers_command">headers command</a>.</p>
<p>Sent when the request process starts replying to the client.</p>
<h3 id="_data">data</h3>
<p>Same as the <a href="../data_command">data command</a>.</p>
<p>Sent when the request process streams data to the client.</p>
<h3 id="_trailers">trailers</h3>
<p>Same as the <a href="../trailers_command">trailers command</a>.</p>
<p>Sent when the request process sends the trailer field values to the client.</p>
<h3 id="_push">push</h3>
<p>Same as the <a href="../push_command">push command</a>.</p>
<p>Sent when the request process pushes a resource to the client.</p>
<h3 id="_switch_protocol">switch_protocol</h3>
<p>Same as the <a href="../switch_protocol_command">switch_protocol command</a>.</p>
<!-- @todo Not done for HTTP/2 yet.-->
<p>Sent when switching to the HTTP/2 or Websocket protocol.</p>
<h2 id="_exports">Exports</h2>
<p>The following function should be called by modules implementing stream handlers to execute the next stream handler in the list:</p>
<ul><li><a href="../cowboy_stream.init">cowboy_stream:init(3)</a> - Initialize a stream
</li>
<li><a href="../cowboy_stream.data">cowboy_stream:data(3)</a> - Handle data for a stream
</li>
<li><a href="../cowboy_stream.info">cowboy_stream:info(3)</a> - Handle a message for a stream
</li>
<li><a href="../cowboy_stream.terminate">cowboy_stream:terminate(3)</a> - Terminate a stream
</li>
<li><a href="../cowboy_stream.early_error">cowboy_stream:early_error(3)</a> - Handle an early error for a stream
</li>
</ul>
<h2 id="_types">Types</h2>
<h3 id="_commands">commands()</h3>
<div class="listingblock"><div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><b><font color="#000000">commands</font></b>() <font color="#990000">::</font> [<font color="#009900">Command</font>]</tt></pre>
</div></div>
<p>See the <a href="../commands">list of commands</a> for details.</p>
<h3 id="_fin">fin()</h3>
<div class="listingblock"><div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><b><font color="#000000">fin</font></b>() <font color="#990000">::</font> <font color="#FF6600">fin</font> | <font color="#FF6600">nofin</font></tt></pre>
</div></div>
<p>Used in commands and events to indicate that this is the end of the stream.</p>
<h3 id="_partial_req">partial_req()</h3>
<div class="listingblock"><div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><b><font color="#000000">req</font></b>() <font color="#990000">::</font> #{
<font color="#0000FF">method</font> <font color="#990000">=></font> <b><font color="#000080">binary</font></b>(), <i><font color="#9A1900">%% case sensitive</font></i>
<font color="#0000FF">version</font> <font color="#990000">=></font> <b><font color="#000000">cowboy:http_version</font></b>() | <b><font color="#000080">atom</font></b>(),
<font color="#0000FF">scheme</font> <font color="#990000">=></font> <b><font color="#000080">binary</font></b>(), <i><font color="#9A1900">%% lowercase; case insensitive</font></i>
<font color="#0000FF">host</font> <font color="#990000">=></font> <b><font color="#000080">binary</font></b>(), <i><font color="#9A1900">%% lowercase; case insensitive</font></i>
<font color="#0000FF">port</font> <font color="#990000">=></font> <b><font color="#000000">inet:port_number</font></b>(),
<font color="#0000FF">path</font> <font color="#990000">=></font> <b><font color="#000080">binary</font></b>(), <i><font color="#9A1900">%% case sensitive</font></i>
<font color="#0000FF">qs</font> <font color="#990000">=></font> <b><font color="#000080">binary</font></b>(), <i><font color="#9A1900">%% case sensitive</font></i>
<font color="#0000FF">headers</font> <font color="#990000">=></font> <b><font color="#000000">cowboy:http_headers</font></b>(),
<font color="#0000FF">peer</font> <font color="#990000">=></font> {<b><font color="#000000">inet:ip_address</font></b>(), <b><font color="#000000">inet:port_number</font></b>()}
}</tt></pre>
</div></div>
<p>Partial request information received when an early error is detected.</p>
<h3 id="_reason">reason()</h3>
<div class="listingblock"><div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><b><font color="#000000">reason</font></b>() <font color="#990000">::</font> <b><font color="#000080">normal</font></b> | <font color="#FF6600">switch_protocol</font>
| {<font color="#FF6600">internal_error</font>, <font color="#FF6600">timeout</font> | {<font color="#FF6600">error</font> | <b><font color="#000080">exit</font></b> | <b><font color="#000080">throw</font></b>, <b><font color="#000000">any</font></b>()}, <font color="#009900">HumanReadable</font>}
| {<font color="#FF6600">socket_error</font>, <font color="#FF6600">closed</font> | <b><font color="#000080">atom</font></b>(), <font color="#009900">HumanReadable</font>}
| {<font color="#FF6600">stream_error</font>, <font color="#009900">Error</font>, <font color="#009900">HumanReadable</font>}
| {<font color="#FF6600">connection_error</font>, <font color="#009900">Error</font>, <font color="#009900">HumanReadable</font>}
| {<font color="#FF6600">stop</font>, <b><font color="#000000">cow_http2:frame</font></b>(), <font color="#009900">HumanReadable</font>}
<font color="#009900">Error</font> <font color="#990000">=</font> <b><font color="#000080">atom</font></b>()
<font color="#009900">HumanReadable</font> <font color="#990000">=</font> <b><font color="#000080">atom</font></b>()</tt></pre>
</div></div>
<p>Reason for the stream termination.</p>
<h3 id="_resp_command">resp_command()</h3>
<div class="listingblock"><div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
http://www.lorenzobettini.it
http://www.gnu.org/software/src-highlite -->
<pre><tt><b><font color="#000000">resp_command</font></b>() <font color="#990000">::</font> {<font color="#FF6600">response</font>, <b><font color="#000000">cowboy:http_status</font></b>(),
<b><font color="#000000">cowboy:http_headers</font></b>(), <b><font color="#000000">cowboy_req:resp_body</font></b>()}</tt></pre>
</div></div>
<p>See the <a href="../response_command">response command</a> for details.</p>
<h3 id="_streamid">streamid()</h3>
<div class="listingblock"><div class="content"><!-- Generator: GNU source-highlight 3.1.8
by Lorenzo Bettini
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http://www.gnu.org/software/src-highlite -->
<pre><tt><b><font color="#000000">streamid</font></b>() <font color="#990000">::</font> <b><font color="#000000">any</font></b>()</tt></pre>
</div></div>
<p>The identifier for this stream.</p>
<p>The identifier is unique over the connection process. It is possible to form a unique identifier node-wide and cluster-wide by wrapping it in a <code>{self(), StreamID}</code> tuple.</p>
<h2 id="_changelog">Changelog</h2>
<ul><li><strong>2.2</strong>: The trailers command was introduced.
</li>
<li><strong>2.0</strong>: Module introduced.
</li>
</ul>
<h2 id="_see_also">See also</h2>
<p><a href="..">cowboy(7)</a>, <a href="../cowboy_http">cowboy_http(3)</a>, <a href="../cowboy_http2">cowboy_http2(3)</a></p>
</div>
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<h3>
Cowboy
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Function Reference
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<li><a href="/docs/en/cowboy/2.3/guide">User Guide</a></li>
<li><a href="/docs/en/cowboy/2.3/manual">Function Reference</a></li>
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